1. Field of the Invention
The invention relates to an exhaust-gas turbine of an exhaust-gas turbocharger connected to an internal-combustion engine and a method for the wet cleaning of its nozzle ring.
2. Discussion of Background
The use of exhaust-gas turbochargers for increasing the output of internal-combustion engines is widespread nowadays. Here, the exhaust gases of the internal-combustion engine are admitted to the exhaust-gas turbine of the turbocharger and their kinetic energy is used to draw in and compress air for the internal-combustion engine. As a function of the actual operating situation and the composition of the fuels used to drive the internal-combustion engine, contamination of the moving blades and nozzle ring occurs in the exhaust-gas turbine sooner or later, the nozzle ring being affected to a substantially greater extent. In heavy-oil operation, a hard layer of contamination forms on the nozzle ring. Such contamination deposits in the region of the nozzle ring lead to a poorer turbine efficiency and consequently to a reduction in the output of the internal-combustion engine. In addition, an increase in the exhaust-gas temperatures and the pressures occurs in the combustion chamber, as a result of which the internal-combustion engine and in particular its valves may be damaged or even destroyed. Therefore the nozzle ring must be regularly freed of the contaminants adhering to them.
Cleaning the nozzle rings in the dismantled state requires the turbocharger to be shut off for a longer period and is therefore undesirable. Consequently, cleaning methods have gained acceptance in which the turbocharger can remain in operation and does not have to be dismantled. Wet cleaning with water and dry cleaning with a granulated material are known as suitable methods of removing nozzle-ring contaminants. The requisite cleaning agent is fed in upstream of the exhaust-gas turbine in the region of the exhaust-gas line connecting the exhaust-gas turbine to the internal-combustion engine.
During the wet cleaning, a large portion of the water used vaporizes on account of the high exhaust-gas temperatures of the internal-combustion engine. Therefore only a portion of the water can be utilized for the cleaning. At full load of a four-stroke internal-combustion engine, the temperatures of the components located at the turbine inlet are above the maximum value admissible for the wet cleaning. In order to avoid thermal damage to the nozzle ring, the moving blades, the cover ring and the turbine casing, the output of the internal-combustion engine has to be reduced before the entry of water into the exhaust-gas turbine. On account of the different expansion of casing and turbine impeller, touching of the turbine impeller at its cover ring may also occur during greater temperature fluctuations. Efficiency losses are associated therewith on the one hand, and unbalance may develop on the other hand. In addition, energy is extracted from the exhaust gases by the vaporization of the water, so that the rotational speed of the exhaust-gas turbine and thus the output of the compressor drop. This is accompanied by an additional decrease in output of the internal-combustion engine.
These disadvantages do not occur during the dry cleaning. However, the use of granulated material may lead to erosion problems in the turbine casing, at the nozzle ring and at the moving blades of the exhaust-gas turbine.
The greatest disadvantage of both methods is the nonuniform distribution of the cleaning agent, for which reason only certain regions of the fixed nozzle ring come in contact with this cleaning agent. Consequently, the contaminants can only be partly removed, so that the cleaning result of these two methods primarily based on the mechanical action of the cleaning agent is inferior to cleaning in the dismantled state. Therefore, although the time intervals up to the next complete cleaning of the nozzle ring can be increased with these solutions, the is dismantling of the turbocharger for cleaning purposes remains imperative.